Not All [tiny house] Walls are Created Equal.

In simple terms, the wall of a house separates inside from outside, protects us from the elements and provides privacy. The walls hold up the roof and the lofts and are a substantial part of a projects material and construction time when all said and done. When those walls are on a mobile platform they have to withstand pretty extreme vibrations and lateral forces that are introduced during transportation and need to be ready for a wide range of potential environments while being as light weight as possible.

So let’s nerd out about the how’s and why of our unique wall assembly and structure for a few minutes. This is meant more as a quick go to resource when fielding the inevitable questions to come and is not an instruction manual or negative commentary on what others have done. This is what we did and why we think we will benefit from those decisions. We will include some links to pertinent past posts and photos but you can view our continually updated VISUAL GALLERY to browse hundreds of build process photos and some videos that will bring you up to speed, starting at day 1.

Before reading this it is important to understand a few things:

The thermal and structural performance of a material and the thermal and structural performance of an assembly are two, very different things.

A Tiny House On Wheels (THOW) is a very different structure than a house on a foundation and the forces on a THOW are a bit different from the forces on a regular sized home and therefore we propose the structure should be considered from a new perspective.

Studs combat gravity loads (vertical) and the sheathing (plywood) combats the lateral (sideways) loads by adding what is called shear strength.

While our final wall assembly design was the result of careful and simultaneous structural AND thermal considerations, we will address each of them separately below.

STRUCTURE

THOW’s must deal aggressively with lateral loads that are amplified by the often narrow and tall proportions and the propensity for vibration and wind forces during transportation. The vertical structure of THOW’s are burdened much more by lateral forces and much less from vertical gravity loads because the total square feet of roof space to bearing wall is proportionally very close compared to a normal sized home with trusses spanning upward of 30-40 feet.

To clarify, let’s look at an occurrence you would find in an average sized home (on a foundation); a trussed roof span of 30’ with trusses installed 24” on center. With a 75 lb roof load, the tributary area of a single truss in this system is 60 square feet, resulting in a 4,500 pound load being supported by that single truss. That load is then split and distributed to its two end bearing points resulting in a 2,250 pound load being supported by the wall studs on each side. This is not considered a big load in the construction industry and is easily supported by standard wall construction.

Now let’s apply this same analysis to our (and many other) tiny houses. We use a 2×6 rafter positioned 2 feet on center, spanning 8’. Under the same 75 lb roof load the 16 sf of tributary area produces a 600 lb point load on each end. This is an incredibly small point load in the world of construction. Now this singular instance of a 600lbs point load is actually pretty conservative (high) especially in our area that uses a code regulated 50 pound per square foot snow load as worst case scenario, And while that point load is pushing the acceptable limits of a single free standing 2×3 stud, the use of frame, skin, paneling and insulation in a unified system increased the strength of the whole. This is a great example of the whole being stronger than the sum of its parts.

If you are still feeling hesitant on our decision to use 2x3s, consider this; When talking about the axial loading of a stud in these instances, we are worried about failure through buckling (bending) not crushing, that is why the acceptable load is increased if the stud, or column is “braced”. Which is what the studs are in the context of a wall with glued and screwed sheathing on each side and rigid foam packed in between.

It was when researching RV’s and their use of a 2×2 structural frame that we began considering the potential for options other than the standard 2×4 THOW framing and what benefits could result in the form of reduced weight, added space and/or improved thermal performance. Clearly, there are a lot of differences between a D.I.Y THOW and a manufactured and engineered RV, but taking inspiration from the RV’s ability to reduce its structural frame and weight by making all of the elements work together as a system; most notably through lamination of the frame and skin was an important moment in our thought process and was the foundation for our first decisions.

Below are the bullet points for our THOW construction decisions as well as a few corresponding photos:

We screwed and glued our skin to our frame on both sides of the wall (our interior plywood paneling and our exterior plywood sheathing) in an attempt to make them act as one.

We used 3/8” plywood instead of ½” plywood sheathing.

We used 2×3 lumber instead of 2×4 lumber for our vertical structure and placed the studs 24” on center instead of 16” on center.

We used double stud corners instead of triple stud corners, eliminated jack studs at openings (instead using header hangers) and we used a single top plate in lieu of a double top plate.

This could be referred to as a form of advanced framing and was first experimented with as a way to reduce lumber (and thus costs) and then adopted by the green building movement who embraced the reduced thermal bridging that comes with that decrease in lumber. For us, reducing the amount of lumber means reducing the amount of weight of our structure and helps us avoid exceeding the 10,000 lb rating of our trailer axles. In reality, even if we had a trailer that could hold 15,000 lbs, we would prefer not to have to pull that much. The results of these decisions also have important thermal performance considerations we will discuss in a bit.

Our roof rafters are spaced 2’ on center, span side to side and are aligned above the vertical studs as required with a single top plate because it would not be able to support potential roof loads if the rafter was bearing at mid span of the top plate between studs.

On the long walls of our house, the sheathing extends up past our top plate and over the ends of our rafters and are glued and screwed to them in order to attach the roof system to the wall system.

We installed heavy duty tension tie down hardware in 8 spots at the base of our walls, including our four corners. This hardware secures the vertical structure to the trailer. We also used small steel angles to secure each stud to the bottom plate. It is important to note that if you only secure the bottom plate to the trailer, you run the risk of your studs separating from your bottom plate during strong racking or uplift forces because the couple of nails or screws used to tack your walls together before lifting them have very little resistance against withdrawal. If you want to learn a little more about our floor system, trailer choice and why it made securing our walls to the trailer so easy and efficient check out THIS POST.

We used slabs of rigid foam, cut on a table saw and installed snugly between studs to hopefully increase the walls rigidity although while this certainly does not hurt, it is unlikely that it will have much structural effect if the walls are constructed properly with a completely adhered skin. It also helped create sturdier feeling 1/4″ interior plywood walls by providing a solid backing support.

We detailed our loft in such a way to prevent the creation of a lateral weak point by eliminating a break in our framing at the loft and running the studs the full height of the wall. We then detailed the connection of the horizontal loft beams to the vertical wall structure in a manner to help resist lateral forces. We have done this by using two beams, one on each side of the stud combined with steel “T” brackets in an attempt to transform a moment in the structure that usually acts as a hinge joint into a more rigid connection by distribute the forces vertically and horizontally along the members, away from the traditional pivot point.

THERMAL PERFORMANCE

This is where the wall assembly really gets interesting in my opinion. One of the biggest critiques of our decision to use 2×3 framing is that we had significantly reduced the wall cavity space for insulation from 3.5” to 2.5” deep. To address this we added an inch of rigid insulation around the exterior of our entire house using what are known as C.I. Panels (continuous insulation). This brought our total thickness of insulation back up to 3.5”. In THIS POST we detailed how we made our own D.I.Y C.I. panels because of our inability to convince professional manufacturers to produce them in the thickness and small quantity we needed.

To better understand why the use of continuous insulation is beneficial, it is important to understand the difference between the performance of a material and the performance of an assembly. If you pack your 2×4 wall cavity (which is 3.5” deep) with rigid foam board rated at R-5 per inch you would assume your insulation value to be 17.5” (+ a little more for sheathing, siding, etc…) This is only partially correct however when you consider that your wood studs have an r-value of only around 1.25 per inch. So while the space between your studs may meet your assumed r-value, the r-value of the entire wall assembly is reduced due to the occurrence of “thermal bridging” which is when unwanted energy takes the path of least resistance through the studs that have a dramatically reduced R-value. To show this using a simple example lets look at a 4×8 section of a standard wall. This section of wall has 4608 square inches of surface area. With studs at 16” on center and at a height of 91.5” plus the bottom plate and double top plate that is around 628 square inches of wood in just that small section of wall. That is 13.6 percent of your wall assembly that is low r-value wood spanning through and bypassing your insulation from the interior paneling all the way to your exterior sheathing and siding. (This number can be higher when you account for doubling up of studs at windows and other overbuilding occurrences)

Our choice to reduce the amount of lumber in our walls in order to save weight now benefits us in a second way by reducing the amount of studs, jack studs, and top plates where thermal bridging would have occurred. On top of this, the use of a continuous insulation around the exterior of our home means that thermal bridging is significantly reduced because our framing is protected from exterior temperatures by an inch of rigid insulation. In a nutshell: less wood means less thermal bridging and more space for insulation and the addition of 1” of continuous insulation results in a wall assembly equal in thickness to that of a 2×4 wall but with better thermal performance because of reduced thermal bridging.

Some other interesting facts about our wall assembly:

Our siding is reclaimed corrugated metal panels that spent the first 50 years of their life on the roof of a local apple bin canopy before being salvaged during structure demolition and incorporated into our tiny house thanks to the help of Steve at LEADING FORCE ENERGY AND DESIGN CENTER. In addition to saving it from the landfill, this material offered a lot of benefits to our project:

The corrugation is only ½” thick from the outside of the concave curve to the outside of its opposite convex curve. Its low profile makes it a good option for a THOW where the builders are trying to maximize the interior space while staying under the 8’-6” max. width regulation (for non-permit travel). The metal panels are also lighter than comparable thickness wood siding.

We were pretty adamant about using a “rain screen” siding system that operates under the assumption that it is not if, but when moisture will make its way behind your siding and therefore takes measures to attempt to create a pressure equalization chamber and to ensure that the water that does make it past the cladding has a path to drain and air has a way to enter and exit to aid in the breathing/drying process. This is usually achieved by leaving a small space between the face of your building paper and the backside of your siding. This gap is often achieved through the addition of furring strips that act as spacers but we were able to utilize the beneficial characteristics of the vertical corrugations intrinsic to our metal panels to have vertical voids behind our siding for water drainage and air circulation.

Additionally these 2’x12’ metal panels are faster to install than smaller plank systems, create less seams, are made from non-organic material (will not rot), and eliminates maintenance as they do not require surface preparation or painting and are non-combustible. We also happen to really like the agriculture industrial aesthetic of this beautiful material that is steeped in Yakima Valley history and is almost symbolic of the valleys sun, sweat and labor, offering an incredible way to tie our project to the agricultural spirit that this city was built on.

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I zoomed in on your first diagram to look at your roof assembly. Are you not venting your roof? I didn’t see any vent space between your roof insulation and your roof sheathing. Unless you added the exterior insulation (as in your walls) outside your roof sheathing, beneath your roof metal. My understanding is that there are two ways of getting out of venting a cathedral roof (according to the IBC): spray in closed cell foam insulation, or insulation ON TOP of your sheathing between roofing and sheathing, in addition to interior insulation. But perhaps you are building more along the lines of an RV and have installed RV roof fan/vents to breathe?

You are correct on all accounts. As you probably know, placing our structure on wheels removes it from the requirements imposed by IBC and IRC. Now this is not to say that we are not trying to comply to the most reasonable extent possible on such a unique project (THOW). We decided against venting our roof and instead choose to do a similar construction to the sprayed in foam option you mentioned which places its emphasis on creating an airtight envelope and controlling/eliminating air movement and thus vapor diffusion from ever entering and becoming a problem. We packed the entire cavity between rafters full of closed cell rigid foam that was cut on a table saw and pressure fit into place. That rigid foam was also fixed to the ceiling using adhesive and we used adhesive caulk around its edges. like all other paneling in SHED the ceiling was then glued into place. The result has been an extremely tight envelope, evidenced by the suction created when opening one of the doors and pressure felt when closing it (It would be really interesting to see what our THOW achieved on a blower door test). Like a few other parts of our build we have tried to research and question archaic or often times misunderstood assumptions and try something new in one of the few real world arenas where that is possible, Tiny Houses. Should our efforts lead to negative consequences we will be sure to update you (the reader) and identify what we would have done differently.
Why not just use spray foam then?
1. It is not as DIY friendly.
2. It is extremely toxic during installation and if it is not done correctly can remain that way many years afterwards.
3. It is expensive, we were quoted around $1.00 per inch, square foot and that that cost would have to be higher if they were to come out and set up everything for such a tiny job.
4. It makes a mess. over spray, under spray, carving away the extra, etc…,

This is a pretty interesting area of building science that is in a state of change as we speak, including the rather recent additions to IBC that for the first time ever to allow a non-vented roof by code. I have linked two of the many articles I found interesting when trying to make our decision.

I commend you on your efforts to experiment with new ideas and methods. I did the same thing when I built my own, 2700 sq.ft. not tiny, house. This was back in 1976 and I was building in rural Tennessee. I did not have to deal with building codes, permits, or any kind of zoning restrictions back then. My framing was standard as was my fiberglass insulation, but then I sheathed the house in 3/8″ plywood like you did and I also covered the sheathing with 1″ closed cell foam board insulation. My family lived happily in this home for 37 years until the last 8 years of obamanomics nearly bankrupted us and we had to sell it.

Couldn’t you put the layer of continuous rigid insulation inside the plywood, or even on the inside of the framing with the same thermal effect break? If you did it inside the framing, you’d even to provide a complete, continuous thermal break on the walls and the roof while maintaining the walls and roof unvented and having a wood decking on the roof (and exterior walls) for standing seam metal or even peel and seal.

Great to see such a well thought out assembly! Building science indeed also benefits tiny houses.

How have you found the impact of the thermal bridge between the trailer’s steel flange and your floor? You brought up the issue briefly in your post on choice of trailer, and I’m wondering to what extent it actually shows itself now that the house has been built. I can see from the assembly drawing that you only have the sill gasket and plywood between the steel, which will effectively be at outside temperature, and your flooring. Any condensation so far?

Thanks, Ren. No condensation issues there or anywhere for that matter. I attribute this to our attention to moisture exhaust using the bathroom fan and range hood during those respective activities as well as the silent but constantly running composting toilet fan which is continuously but imperceptibly exhausting any remaining moisture and achieving a preferable hourly air exchange. With that said there is definitely a noticeable to the touch floor temperature difference around the perimeter where that condition exists. Luckily/by design a majority of the perimeter of our home/floor plan are covered with built-ins and we rarely interact with (stand on) the edge few inches that are colder.

I think I’ve read through almost your entire website. Thank you for providing such thorough, thought out explanations for all of your building choices! Your build is very inspiring

Did you consider using your interior plywood as your only sheathing? You could have used 3/8″ ADX plywood on the inside of the studs, wrapped the house in continuous XPS without laminating it to 3/8″ CDX, and saved a little weight and effort.

I would love to hear your thoughts on this, thanks again for all the resources you’ve provided future builders 🙂

Sean, Sorry for missing your question until Paulina above tipped me off! It is always great to hear that people are consuming our information and find it useful.

We hadn’t really considered that option. It may have been an oversight on our part once we got fixated on those ‘C.I. Panels.’ You bring up a great option. We did like the idea of having a full wood diagram screwed and glued to both sides of the stud bay for increased strength, but let in bracing or metal strapping (linked to above in Paulina’s question) could have taken care of that on the exterior.

Man that would have saved a lot of effort laminating those stupid things…:)

I have a similar question to Sean’s. I plan on using fibrous board continuous insulation (not structural) on the outside of my 2×4 studs. Then adding horizontal 2×2 battens to the interior of the studs to create a wiring cavity and reduce thermal bridging. Then using 7/16″ plywood as my interior sheathing/finish. I’m wondering if this system could eliminate the need for exterior sheathing, between my studs and the fibrous board insulation (Gutex).
Thanks for all the thorough explanations!

Thanks for tipping me off that I missed Sean’s question 🙂 As for your question, your continuous insulation should really take care of the thermal bridging pretty well and you could eliminate the interior battens, apply the interior 7/16 sheathing to the studs and wire your electric through the stud cavity (which is standard in regular homes). This would reduce the thickness and increase the strength of your wall. But yes, even with the method you describe above you can get your shear strength in other ways. here is an article that is right up your ally: http://www.finehomebuilding.com/2011/05/19/4-options-for-shear-bracing-foam-sheathed-walls

Thanks so much for the response! And the article link, which is also very helpful. I’m probably going way overboard, but the idea for the horizontal battens and separate wiring cavity is to have a continuous vapor permeable air barrier membrane between them and the exterior studs. This limits the number of penetrations through the air barrier, while locating it as far to the interior as possible. I’m thinking the battens will also work as additional bracing? Still thinking this through…:)

I had a 500sf cabin build in my property I’m having condensation build ul around the bottom of the structure it’s built on poles but the did lateral framing in part of it and it seems to be trapping the moisture

Thank you so much for all of the energy you have put into this blog. I was curious what you think about using actual 2×3 ( 1.5 x 3 ) to achieve a similar result? Also how tall were your walls? In AK the road height max is 15 feet. I plan to do a shed roof with the tall wall at 12 foot. Im slightly intimidated to experiment with this technique. Any thoughts?

A bigger sized board certainly only helps strength (but adds weight). You could always do a 2×4 or 2×6 wall on just the tall wall if you’re worried. I also like the idea (and almost implemented it) of using temporary turnbuckles for x-bracing during transportation. Backcountry tiny homes now does this with some of their builds. It adds piece of mind during transportation but are removable (including the anchor plates) when you’re living in it.